Three-dimensional fabrication of functional single crystal waveguides inside glass by femtosecond laser irradiation

Ultrafast femtosecond (fs) pulsed lasers are capable of inducing a wide range of local structural modifications inside transparent materials, enabling direct three dimensional (3D) patterning of features inside monolithic samples. As such, direct laser-writing of waveguides, gratings, and other optical components has attracted considerable interest for the purpose of 3D optical integration. Much progress has been made in writing index-gradient waveguides in glass, but such amorphous waveguides are inherently passive structures. For the important class of active optics applications that require a second-order nonlinear optical response, glasses are fundamentally unsuitable due to their isotropically disordered structure. Bulk single crystals of non-centrosymmetric phases are typically used when such functionality is needed, but by choosing specific glass compositions which crystallize into these phases, nonlinear optical properties may be introduced locally into a glass through laser-induced crystallization. In this dissertation, major issues of practical and theoretical importance for fs laser crystallization are examined in the model LaBGeO5 glass system–including focal depth effects, nucleation mechanisms, growth dynamics, and obtained morphologies–in order to develop a model framework that guides the optimization of process parameters for obtaining high quality single crystal waveguides with functional capability. Initiation of new crystals from bulk glass in this strongly glass-forming system involves a complex multi-step process with a strong sensitivity to focal depth. The key mechanism is identified as a reliance on heterogeneous nucleation to increase the nucleation rate to practical laboratory timescales, which is facilitated by the formation of free surface in the form of bubbles and the local composition modification induced by the laser heating. The role of focal depth is to modulate the heat source geometry through optical aberration and thereby influence the geometry of the melt, the bubble distribution, and the element redistribution. The convergence of local composition and local temperature at the bubble surface ultimately determines the nucleation rate, so focal depth may be used as a process parameter to accelerate crystallization.If consistent heating conditions are desired at different focal depths, which would generally be the case for 3D fabrication, aberration effects are detrimental. A novel method is described for correcting aberration effects when irradiating through multiple refracting layers in order to produce consistent focal conditions at arbitrary focal depths inside externally heated samples inside a closed furnace. This enables simultaneous aberration correction and in-situ annealing, which is essential for the suppression of cracks.Patterning of continuous crystal features like waveguides requires scanning the focus through the glass. Counterintuitively, a preferential and seemingly consistent lattice orientation with respect to the scan direction is found to be associated with polycrystallinity rather than single-crystallinity, as has generally been thought. Rather, the retention of preferential orientation even across changes in scan direction arises from directional filtering and competitive maximization of growth rate between grains of multiple orientations, of which new instances are frequently attempted at the growth front. Crystal lines exhibiting preferential orientation may thus contain many similarly-oriented but distinct grains separated by low-angle grain boundaries, and these would generally be overlooked by low angular resolution optical methods of assessment.Nevertheless, irradiation conditions which are capable of suppressing this polycrystallinity are identified and explained in terms of the collective interactions between the laser-induced temperature gradient, the focal scan rate, the intrinsic temperature and orientation dependences of crystal growth rate, and the tendency of the growth front to initiate competing grains. Single crystallinity of lines written under these conditions is confirmed by high resolution electron backscatter diffraction, and a model of the dynamics of fs laser-induced single-crystal growth is presented. Finally, the waveguiding capability of fs laser-written single crystal lines inside a glass is demonstrated and quantified for the first time. A substantial power transmission is obtained in the case of the waveguide with the most consistent long-range uniformity. This confirms the potential applicability of the technique for writing nonlinear optical crystal waveguides, which until now has been largely hypothetical. This work thus provides the proof-of-concept for three dimensional fabrication of functional single-crystal waveguides inside glass